1. Field of the Invention
The invention pertains to an x-ray examination apparatus including an x-ray source, an x-ray image intensifier for converting an x-ray image made by irradiating an object with an x-ray beam from the x-ray source into a visible image and an image detection device for detecting said visible image, the image detection device being provided with an auxiliary light-detection system for performing brightness-control for said visible image, which includes a photodetector for generating a photodetector signal.
2. Description of the Related Art
An x-ray examination apparatus of said kind is known from the European Patent Application EP 0 217 456 which corresponds to U.S. Pat. No. 4,809,309.
The known x-ray examination apparatus is fitted with an auxiliary light-detection system having a photodetector being a charge-coupled device (CCD-detector). Brightness-values in the visible image are converted into an electronic photodetector signal by the photodetector. An advantage of providing the auxiliary light detection system with a CCD-detector that has a multitude of photosensitive elements, is that a CCD-detector has spatial resolving power, so that brightness control is carried out using spatial information acquired by an auxiliary light-detection system as described in the cited reference. The auxiliary light-detection system employs the CCD-detector for producing an exposure-control signal for controlling exposure in the visible image, e.g. by adjusting the x-ray source. A CCD-detector, as incorporated in the known auxiliary light-detection system, however, inherently has a comparatively limited dynamic range of brightness for which it operates adequately. Within a single x-ray image the brightness-values may have a range of typically 15 dB, that is sufficiently limited for employing a CCD-detector in the auxiliary light-detection system. Within a single x-ray image the dynamic range remains limited due to reduction of contrast by scattering processes in the x-ray image intensifier. Among various x-ray images in a collection of x-ray images the average brightness-values appear to vary over a much wider range, typically 50 dB in practical examinations. Such a collection is formed e.g. either by making a sequence of x-ray exposures or by x-ray exposures made during various x-ray examinations each of them being performed at a different x-ray dose rate. Therefore, the use of a CCD-detector per se as a photodetector in an auxiliary light-detection system for brightness control has a drawback in that the range of sensitivity only limitedly matches the dynamical range of brightness of images in a collection of x-ray exposures.